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Geometry Terms: Chapter 1 Acute Angle An angle with degree measure between 0 and 90. Adjacent Angles Two angles that lie in the same plane, have a common vertex, and a common side, but no common interior points. Angle bisector A ray that divides an angle into to congruent angles Between/ Betweenness of points For any two points A and B on a line, there is another point C between A and B if and only if A, B, and C are collinear and AC + CB = AB. Collinear Points that lie on the same plane Complementary Angles Two angles with measures that have a sum of 90. Concave A polygon for which there is a line containing a side of the polygon that also contains a point in the interior of the polygon. Congruent Having the same measure. Construction A method of creating geometric figures without the benefit of measuring tools. Generally, only a pencil, straightedge and compass are used. Convex A polygon for which there is no line that contains both a side of the polygon and a point in the interior of the polygon. Coplanar Points that lie in the same plane. Degree A unit of measure used in measuring angles and arcs. An arc of a circle with a measure of 1degree is 1/ 360 of the entire circle. Exterior A point is in the exterior of an angle if it is neither on the angle nor in the interior of the angle. Interior A point is in the interior of an angle if it does not lie on the angle itself and it lies on a segment with endpoints that are on the sides of the angle. Line A basic undefined term of geometry. It is made up of points and has no thickness or width. In a figure, it is shown with an arrowhead at each end. Usually named by lowercase script letters or by writing the capital letters for two endpoints, with a double arrow over the pair of letters. Line Segment A measureable part of a line that consists of two points, called endpoints, and all of the points between them. Linear Pair A pair of adjacent angles whose non- common sides are opposite rays. Locus The set of points that satisfy a given condition. Midpoint The point halfway between the endpoints of a segment. N-gon A polygon with n sides. Obtuse Angle An angle with degree measure greater that 90 and less than 180. Opposite rays Two rays BA and BC such that B is between A and C. Perimeter The sum of the lengths of the sides of a polygon. Perpendicular Lines that form 4 right angles. Plane A basic undefined term in geometry. It is a flat surface made up of points that has no depth and extends indefinitely in all directions. In a figure, it is often represented by a shaded, slanted 4sided figure. Usually named by a capital script letter or by three non collinear points. Point A basic undefined term in geometry. It is a location. In a figure, it is usually represented by a dot and named by a capital letter. Polygon A closed figure formed by a finite number of coplanar segments called sides such that the following conditions are met: 1. The sides that have a common endpoint are noncollinear. 2. Each side intersects exactly two other sides, but only at their endpoints, called vertices. Precision Depends on the smallest unit available on the measurement tool. Ray A part of a line. It has one endpoint and extends indefinitely in one direction. Regular Polygon A convex polygon in which all of the sides are congruent and all of the angles are congruent. Relative Error The ratio of the half-unit difference in precision to the entire measure, expressed as a percent. Right Angle An angle with a degree measure of 90. Segment Bisector A segment, line, or plane that intersects a segment at its midpoint. Sides What rays that make up angles are caled. Space A boundless 3-D set of all points. Supplementary Angles Two angles with measures that have a sum of 180. Undefined Terms Words, usually readily understood, that are not formally explained by means of more basic words and concepts. Includes: point, line, and plane. Vertex A common endpoint. Vertical Angles Two nonadjacent angles formed by two intersecting lines.